1. Field of the Invention
The present invention relates to an electronic part of surface mounted type which is constituted by a chip, such as a diode device and a transistor device, molded in resin and a method of fixing such an electronic part onto a circuit board.
2. Description of the Prior Art
FIG. 1 shows a conventional structure of a two-terminal diode part exemplifying an electronic part of surface mounted type. As shown in FIG. 1, in a conventional diode part 10, a chip 12 of a diode device is fixed onto a mount portion 11a, and the chip 12 and two lead terminals 11b which are external lead portions are electrically connected by, for example, bonding wires 13. Under this condition, the mount portion 11a, the chip 12, and a base end portion of each of the lead terminals 11b are molded in a resin 14. A portion of each of the lead terminals 11b which is protruding out of the resin 14 is bent downward. Since the molding of the resin 14 is performed by use of a metal mold, the resin 14 is formed into a rectangular parallelepiped block form having a flat bottom surface.
As a method of mounting the part 10 thus molded in resin onto a circuit board, a so-called wave soldering method is widely used in which a portion required to be connected is dipped in a liquid wave solder. FIGS. 2A to 2D sequentially show an example of a process of fixing the diode part 10 of the above-described conventional structure onto a circuit board by the wave soldering method.
First, as shown in FIG. 2A, an adhesive agent 16 is applied onto a predetermined fixing position on a circuit board 15. Then, as shown in FIG. 2B, the bottom surface of the resin 14 is placed onto the adhesive agent 16, and by hardening the adhesive agent 16 under this condition, the diode part 10 is auxiliarily fixed onto the circuit board 15. Under this condition, the lead terminals 11b are normally in contact with a circuit pattern (not shown) formed on the circuit board 15.
After the pretreatment has been completed in which the application and hardening of the adhesive agent are performed, by dipping the diode part 10 into wave solder in a solder bath 17 under a condition where the circuit board 15 is being turned upside down as shown in FIG. 2C, the diode pattern 10 is fixed under a condition where the lead terminals 11b are electrically connected onto the circuit pattern on the circuit board 15 as shown in FIG. 2D.
In fixing the diode part 10 onto the circuit board 15 by the above-described wave soldering method, if the resin 14 and the adhesive agent 16 are not completely fixed in the auxiliary fixing in the pretreatment shown in FIGS. 2A and 2B, the diode part 10 may be fall off from the circuit board 15 when the circuit board 15 is turned upside down in the dipping treatment into the solder bath 17 shown in FIG. 2C.
Such a falling off of the diode part 10 from the circuit board 15 due to incomplete fixing results from a fact that fat and oil included in the resin 14 constituting the fixing surface of the diode part 10 deteriorates the adhesive strength of the adhesive agent 16.
Conventionally, the incomplete fixing has been prevented by taking the following measures:
(i) fat and oil included in the surface of the resin is removed by a so-called burning method in which the surface of the resin is burned by putting the surface into flames prior to the fixing; PA1 (ii) an adhesive agent is used whose adhesive strength is not deteriorated by fat and oil; and PA1 (iii) fat and oil is also removed when refuse of the resin produced in the resin molding is removed by use of a sandblaster.
In a case where the burning method mentioned in (i) is used, however, it is necessary to install equipment for the burning treatment and peripheral equipment such as an exhauster for exhausting smoke generated in the burning treatment, which results in an increase of the cost of equipment. Moreover, in the burning method, since flames are used, the degree of safety is low, and there is danger in operating a flame generator. Further, in a case where a limited region of a narrow surface such as the bottom surface of the diode part 10 is burning-treated, it is difficult to perform control such as specification of a treatment region, and in addition, if the flames are not appropriately controlled, there are cases where incomplete fixing and short circuit occur. Hence, in actuality, it is extremely difficult to use the burning method.
The adhesive agent mentioned in (ii) whose adhesive strength is not deteriorated by fat and oil is desirable in that the falling off of the part can be prevented. However, since such an adhesive agent is extremely expensive compared to normally-used adhesive agents, the use thereof will largely increase the cost of the electric part. From this, it can be regarded as an impractical method.
The sandblaster mentioned in (iii) is originally used for removing refuse of resin. It is therefore difficult to specify the treatment region on the bottom surface of the resin and to control the degree of treatment. Hence, it is not suitable for processing which requires accuracy such as the surface treatment of electronic parts of this type.